1. Field of the Invention
The present invention relates to the treatment of gases containing sulfur dioxide, and more particularly, to an improvement in those processes for removal of sulfur dioxide which use an aqueous absorbent containing a sulfite-forming additive in approximately the amount required to convert the sulfur dioxide to sulfite. The term "sulfite" as used herein refers to both the sulfite and bisulfite form of the anion.
2. Description of the Prior Art
The art is extensive on sulfur dioxide removal from flue gases by means of aqueous absorbents containing sulfite-forming additives. The sulfite-forming additives that are generally considered for the removal of sulfur dioxide are such materials as the oxides, hydroxides and carbonates of alkali and alkaline earth metals, as well as ammonia and its derivatives. The absorption of sulfur dioxide (SO.sub.2) by suitable contact of an SO.sub.2 -containing gas phase with an aqueous solution or slurry containing such additives is both rapid and efficient, provided certain conditions are satisfied. Efficient absorption depends upon operating with scrubbing solutions which contain a low equilibrium vapor pressure of SO.sub.2 over them. Such a condition is reached when the concentration of sulfite is relatively low, i.e. less than about five weight percent, and when the pH is relatively high, i.e. greater than about 5.5-6.0.
Rapid absorption can be achieved, in general, by operating with scrubbers of proper design which provide adquate contact area between the liquid and the gas and in which turbulent flow conditions are maintained.
The state of the art of sulfur dioxide removal from industrial waste gases as of the year 1958 is presented by the Bureau of Mines (U.S. Department of the Interior) in its Information Circular 7836 entitled "Sulfur Dioxide - Its Chemistry and Removal from Industrial Waste Gases" by D. Bienstock, L. W. Brunn, E. M. Murphy and H. E. Benson.
The difficulties associated with the use of the above-cited absorbents include the following. Firstly, the perhaps most serious, is the formation of sulfates by oxidation of the sulfites formed in the scrubber. Presumably, such sulfates result from reaction of the oxygen contained in the flue gas with the sulfites, either sulfite or bisulfite. In any case, the sulfate content of the absorbent builds up in any regenerative system, that is one where the additive is regenerated for reuse in the scrubber. To prevent such build-up, a portion of the absorbent must be discarded, thus causing a waste of useful absorbent. Secondly, the regeneration of the additive has always been a problem in terms of cost. Any regeneration procedure which is effective but cheaper than those now known will be an improvement. Thirdly and lastly, the concentration of the additive in aqueous absorbents of the foregoing type is very small since the low SO.sub.2 content of the gas requires a correspondingly small amount of additive. Yet adequate contact between gas and additive requires a relatively large volume of aqueous absorbent. Such dilute aqueous solutions impose an undesirably low temperature of operation in the scrubber. Higher concentrations of salts in the aqueous absorbent would permit higher temperature scrubbing, and hence less reheat is required before the scrubbed gas is released to the atmosphere.
The primary object of the present invention is to provide an improvement in the processes of the type described which alleviates these problems. Patents which may be considered pertinent to out improvement, although not, in our opinion, suggestive thereof are: U.S. Pat. Nos. 1,795,121 and 3,523,755, and British Patents Nos. 134,943 and 459,418.